Dr Mei Ting Mak

We present the results from the simulations of the atmospheres of hot-Jupiters HD 189733b, HD 209458b, and WASP-39b, assuming the presence of three different types of haze. Using a 3D general circulation model, the Unified Model, we capture the advection, settling, and radiative impact of Titan-, water-world-, and soot-like haze, with a particle radius of 1.5 nm. We show that the radiative impact of haze leads to drastic changes in the thermal structure and circulation in the atmosphere. We then show that in all our simulations, (1) the super-rotating jet largely determines the day-to-night haze distribution, (2) eddies drive the latitudinal haze distribution, and (3) the divergent and eddy component of the wind control the finer structure of the haze distribution. We further show that the stronger the absorption strength of the haze, the stronger the super-rotating jet, lesser the difference of the day-to-night haze distribution, and larger the transit depth in the synthetic transmission spectrum. We also demonstrate that the presence of such small hazes could result in a stronger haze opacity over the morning terminator in all three planets. This could lead to an observable terminator asymmetry in WASP-39b, with the morning terminator presenting a larger transit depth than the evening terminator. This work suggests that, although it might not be a typical detection feature for hot Jupiters, an observed increase in transit depth over the morning terminator across the ultraviolet and optical wavelength regime could serve as a strong indicator of the presence of haze.

Rocky planets orbiting M-dwarf stars are prime targets for characterizing terrestrial atmospheres, yet their long-term evolution under intense stellar winds and high-energy radiation remains poorly understood. The Kepler-1649 system, which hosts two terrestrial exoplanets orbiting an M5V star, presents a valuable opportunity to explore atmospheric evolution in the extreme environments characteristic of M-dwarf stellar systems. In this Letter we show that both planets could have retained atmospheres over gigayear timescales. Using a multi-species magnetohydrodynamic model, we simulate atmospheric ion escape driven by stellar winds and extreme ultraviolet radiation from 0.7 to 4.8 Gyrs. The results show that total ion escape rates follow a power-law decline (∝ τ ^−1.6 for Kepler-1649 b, ∝ τ ^−1.5 for Kepler-1649 c ), with O+ dominating atmospheric loss (76.8%–98.7%). The escape rates at 4.8 Gyrs are two orders of magnitude lower than those during the early epochs (1.9 × 1027 s^−1 at 0.7 Gyr vs. 3.0 × 1025 s^−1 at 4.8 Gyrs for planet b ), while planet b consistently exhibits 1.1–1.9× higher O+ escape rates than planet c due to its closer orbit (0.051 AU vs. 0.088 AU). Despite substantial early atmospheric erosion, both planets may still retain significant atmospheres, suggesting the potential for long-term habitability. These findings offer predictive insight into atmospheric retention in M-dwarf systems and inform future JWST observations aimed at refining habitability assessments.

In order to assess observational evidence for potential atmospheric biosignatures on exoplanets, it will be essential to test whether spectral fingerprints from multiple gases can be explained by abiotic or biotic-only processes. Here, we develop and apply a coupled 1D atmosphere-ocean-ecosystem model to understand how primitive biospheres, which exploit abiotic sources of H2, CO, and O2, could influence the atmospheric composition of rocky terrestrial exoplanets. We apply this to the Earth at 3.8 Ga and to TRAPPIST-1e. We focus on metabolisms that evolved before the evolution of oxygenic photosynthesis, which consume H2 and CO and produce potentially detectable levels of CH4. O2-consuming metabolisms are also considered for TRAPPIST-1e, as abiotic O2 production is predicted on M-dwarf orbiting planets. We show that these biospheres can lead to high levels of surface O2 (approximately 1–5 per cent) as a result of CO consumption, which could allow high O2 scenarios, by removing the main loss mechanisms of atomic oxygen. Increasing stratospheric temperatures, which increases atmospheric OH can reduce the likelihood of such a state forming. O2-consuming metabolisms could also lower O2 levels to around 10 ppm and support a productive biosphere at low reductant inputs. Using predicted transmission spectral features from CH4, CO, O2/O3, and CO2 across the hypothesis space for tectonic reductant input, we show that biotically produced CH4 may only be detectable at high reductant inputs. CO is also likely to be a dominant feature in transmission spectra for planets orbiting M-dwarfs, which could reduce the confidence in any potential biosignature observations linked to these biospheres.

Using a 3D General Circulation Model, the Unified Model, we present results from simulations of a tidally locked TRAPPIST-1e with varying carbon dioxide CO2 and methane CH4 gas concentrations, and their corresponding prescribed spherical haze profiles. Our results show that the presence of CO2 leads to a warmer atmosphere globally due to its greenhouse effect, with the increase of surface temperature on the dayside surface reaching up to ∼14.1 K, and on the nightside up to ∼21.2 K. Increasing presence of CH4 first elevates the surface temperature on the dayside, followed by a decrease due to the balance of tropospheric warming and stratospheric cooling. A thin layer of haze, formed when the partial pressures of CH4 to CO2 (pCH4/pCO2) = 0.1, leads to a dayside warming of ∼4.9 K due to a change in the water vapour H2O distribution. The presence of a haze layer that formed beyond the ratio of 0.1 leads to dayside cooling. The haze reaches an optical threshold thickness when pCH4/pCO2 ∼ 0.4 beyond which the dayside mean surface temperature does not vary much. The planet is more favourable to maintaining liquid water on the surface (mean surface temperature above 273.15 K) when pCO2 is high, pCH4 is low, and the haze layer is thin. The effect of CO2, CH4, and haze on the dayside is similar to that for a rapidly rotating planet. On the contrary, their effect on the nightside depends on the wind structure and the wind speed in the simulation